Engineering ZrO2–Ru interface to boost Fischer-Tropsch synthesis to olefins

Understanding the structures and reaction mechanisms of interfacial active sites in the Fisher-Tropsch synthesis reaction is highly desirable but challenging. Herein, we show that the ZrO2-Ru interface could be engineered by loading the ZrO2 promoter onto silica-supported Ru nanoparticles (ZrRu/SiO2), achieving 7.6 times higher intrinsic activity and ~45% reduction in the apparent activation energy compared with the unpromoted Ru/SiO2 catalyst. Various characterizations and theoretical calculations reveal that the highly dispersed ZrO2 promoter strongly binds the Ru nanoparticles to form the Zr-O-Ru interfacial structure, which strengthens the hydrogen spillover effect and serves as a reservoir for active H species by forming Zr-OH* species. In particular, the formation of the Zr-O-Ru interface and presence of the hydroxyl species alter the H-assisted CO dissociation route from the formyl (HCO*) pathway to the hydroxy-methylidyne (COH*) pathway, significantly lowering the energy barrier of rate-limiting CO dissociation step and greatly increasing the reactivity. This investigation deepens our understanding of the metal-promoter interaction, and provides an effective strategy to design efficient industrial Fisher-Tropsch synthesis catalysts.


Definitions of catalytic activity and selectivity
CO conversion was calculated according to the following equation: Where COinlet and COoutlet refer to the molar numbers of CO at the inlet and outlet, respectively.
Product selectivity was calculated according to the following equation: where Si denotes the selectivity to product i on a carbon basis, Ni is the molar fraction of product i, and ni is the carbon number of product i.The selectivity of oxygenates was less than 1% and has been excluded from the reported product selectivity.
The ruthenium-weight-based activity (ruthenium time yield, RuTY) was calculated as following: Where WHSV is the weight hourly space velocity, XCO is the CO conversion, CO Concentration represents the molar concentration of CO in the feedstock, ξRu represents the loading amount of Ru measured by XRF.
The turnover frequency (TOF) value was calculated as following: Where MRu is the relative atomic mass of Ru (101.07 g•mol -1 ), DRu is the dispersion of metallic Ru measured by CO chemisorption.
The space time yield of olefins product was calculated as following: Where   is the mass of olefins product produced within a certain reaction time (t),  .is the mass of catalyst packed in the reactor.
The chain growth probability (α) was calculated according to Anderson-Schulz-Flory distribution: where n is the carbon number of products, Wn is mass fraction of the hydrocarbons with a carbon number of n, α is chain growth probability.
The equation of ( 4) can be edited as: Plotting Ln(Wn/n) versus n (carbon number), and the chain growth probability (α) can be obtained by calculating the slope (Lnα).

Supplementary Catalyst Characterization
The element content of various samples was measured by an energy dispersive X-ray fluorescence (XRF, Rigaku ZSX Primus II) and ICP-OES (Varian ICP-OES 720).
The specific surface area, pore volume, and the average pore diameter of the samples were measured by N2 physisorption on Micromeritics 2020 instrument working at -196 °C.Prior to N2 adsorption, the samples were degassed under vacuum at 300 °C for 10 h.Brunauer-Emmett-Tellwer (BET) method was used to calculate the specific surface areas.Pore volume and pore size were determined by the Barrett-Joyner-Halenda (BJH) method.
Transmission electron microscopy (TEM) and high-solution transmission electron microscopy (HRTEM) measurements were conducted on a FEI Tecnai G2 F20 S-TWIN equipment with 200 kV accelerating voltage.The nanoparticle size distribution for each sample was determined using at least 200 nanoparticles.
The size distribution of metallic Ru was analyzed based on the statistics of over 200 particles.The dispersion of metallic Ru nanoparticles (  ) was further calculated from the following equation: where  denotes the volumes of a Ru atom in the bulk of the metal (Å 3 ),  denotes the specific surface area of a Ru atom (Å 2 ),  denotes the mean particle size (nm),  denotes the Ru atomic mass (g•mol -1 ),   denotes the mean number of atoms in the exposed surface area (m -2 ),  denotes the mass density of metallic Ru (g•cm -3 ), and   is Avogadro constant (6.02 × 10 23 mol -1 ).For hcp Ru exposed (001) surface, the above equation can be converted to Temperature programmed reduction with hydrogen (H2-TPR), temperature Hydrogen spillover detection by the WO3 powder experiment was performed in a quartz reaction tube at 100 °C. 1 g of WO3 and 0.02 g of catalyst samples were placed in a quartz reaction tube and held in place using silica wool.Then, the tube temperature was controlled to be 100 °C in a furnace, and H2 was introduced into the reaction tube at a consistent rate of 50 mL•min -1 .After 60 min, the H2 flow was switched to Ar and cooled to room temperature.Finally, the reaction tube was removed, and color changes in the WO3 powder were observed and recorded.
Due to the low water-gas shift activity of Ru-based catalysts, CO2 selectivity remains almost negligible for both Ru/SiO2-0Na and Zr-Ru/SiO2-0Na catalysts.
It is evident that introducing the Zr promoter significantly enhances the intrinsic activity of the Ru/SiO2 catalyst.) Supplementary Fig. 7. XRD patterns for the reduced catalysts of Ru/SiO2-0Na and Zr-Ru/SiO2-0Na.

Notes:
XRD patterns of the reduced catalysts without Na promoter were shown in Supplementary Fig. 7.It can be observed that adding the Zr promoter does not affect the phase of the catalysts, which remains metallic Ru (JCPDS, 06-0663).
Furthermore, no diffraction peaks for the crystalline phase of the ZrO2 promoter were observed.

Notes:
Comparison of the average particle sizes of metallic Ru nanoparticles measured by TEM for all reduced samples indicates that the ZrO2 loading exhibits little influence on the size of metallic Ru nanoparticles (Supplementary Fig. 8).The dispersion of metallic Ru nanoparticles, calculated based on the average particle size obtained from the TEM results, is determined to be 16.7% and 17.7% for Ru/SiO2-0Na and Zr-Ru/SiO2-0Na, respectively.
programmed desorption of H2 (H2-TPD) and temperature-programmed surface reaction (TPSR) of CO measurements were all carried out on a chemisorption apparatus (Micromeritics 2920 Auto Chem II) with a thermal conductivity detector (TCD) and MKS Cirrus 2 mass spectrometer (MS).Before TPR measurement, the samples were pretreated in He flow at 120 °C for 60 min, then cooled down to 50 °C.The reactor was heated to 800 °C at a heating rate of 10 °C•min -1 in 5 vol% H2 (balance Ar, 30 mL•min -1 ).The signal of H2 consumption was monitored by TCD.The H2 temperature programmed desorption (H2-TPD) measurements were performed on fresh catalysts.The 100 mg of sample was first reduced at 450 °C for 2 h under flowing H2.H2 adsorption was employed at 50 °C for 30 min by passing H2 with a flow rate of 30 mL•min - 1 .The TPD signal was recorded by TCD and the signals of m/z = 2 (H2) was monitored by mass spectrometer with linear temperature increase up to 800 °C at a ramp rate of 10 °C•min -1 .For the temperature-programmed surface reaction of CO (CO-TPSR), the sample was pretreated in a H2 flow at 450 °C for 2 h and then flushing with He for 30 min.The temperature was cooled down to 50 °C and CO flow was switched to realize the saturated CO adsorption.The sample was heated up to 500 °C at 10 °C•min -1 in flowing of H2 flow, and the signals of m/z=16 (CH4) was monitored by mass spectrometer.

Supplementary Fig. 9 .Supplementary Fig. 15 .Supplementary Fig. 16 . 2 Supplementary Fig. 17 .
(a) N2 adsorption-desorption isotherms for various xZr/Ru samples; (b) XRD patterns for the fresh catalysts; (c) XRD patterns for the spent catalysts.(▲ represents the diffraction peaks of diluent SiO2.) spent 0.5Zr/Ru catalyst.Ru is depicted in green, Si in yellow, Zr in red.The orange arrow represents the line-scanning direction.The related STEM-EDS line-scanning results are displayed in the insets.Ru K-edge XANES spectra of the reduced xZr/Ru samples.(a) Normalized Ru K-edge k2-weighted EXAFS spectra of the reduced and spent 0.5Zr/Ru samples.(b) Experimental (colored lines) and best fit (dashed black lines) FT-EXAFS spectra for the reduced and spent 0.5Zr/Ru samples measured at the Zr K-edge.Structure determination of the Zr promoter.(a) XANES partial enlarged detail enhancing the white-line peak feature for the reduced xZr/Ru (x = 0.2, 0.5, and 1) samples.(b) Normalized Zr K-edge k2weighted EXAFS spectra of the xZr/Ru samples.dissociation and hydrogen atoms spillover on (ZrO2)3/Ru (001).Ru (dark green spheres), Zr (light green spheres), O (red spheres), and H (white spheres) atoms are shown.Supplementary Fig. 27.Energy profiles and the corresponding structures in the process of CO direct dissociation on Ru (001) and (ZrO2)3/Ru (001) surface, respectively.Ru (dark green spheres), Zr (light green spheres), O (red spheres), and C (gray spheres) atoms are shown.
The top view of optimized structure for Zr-O-Ru interfacial structure (a) and pure metallic Ru surface (b) with higher H coverage.Ru (dark green spheres), Zr (light green spheres), O (red spheres) atoms, and H (white spheres) atoms are shown.Energy profiles of different intermediates and COH* dissociation over Zr3O6H5/Ru (001) surface.Red solid line represents the energy profile of COH* dissociation to C* and OH* pathway and blue solid line represents the energy profile of COH* hydrogenation to HCOH* dissociation pathway.Energy profiles for the hydrogenation of the OH* species to H2O molecules the further desorption of H2O molecules at the Ru sites of Ru (001) (top)and the Zr sites of Zr3O6H5/Ru (001) (bottom).